Polybenzimidazole is polymeric material having the highest level of heat resistance, strength and chemical stability and was used in many applications as heat-resistant fiber, shaped part, coating varnish, etc.
However, the polybenzimidazole currently available on the market has high content of metals such as iron, chromium, nickel and copper and this has significantly limited their use as parts in the manufacture of semiconductor and display devices which are greatly influenced by metallic impurities in terms of product characteristics.
The sole commercial supplier of polybenzimidazole is Hoechst Celanese, U. S. A., who is manufacturing their product by solid-state polymerization method without using polymerization solvent. The present inventor conducted intensive studies in order to locate the cause of the significant presence of metallic impurities in polybenzimidazole. As a result, the inventor found that the metallic material used in the production equipment wore out in the process of production due to high strength of polybenzimidazole and that the resulting contamination of the product polybenzimidazole with the worn material was the cause of the high metallic concentration.
Even if the polybenzimidazole material obtained as a powder by means of above-mentioned process is treated by the following procedure, it is difficult to reduce the total metallic concentration except alkaline metal and alkaline earth metal to a sufficiently low level that satisfies the requirement for use as parts in the manufacture of semiconductor and display devices which are largely influenced by metallic impurities in terms of characteristics. The procedure comprises the steps of solubilizing the polybenzimidazole powder to separate the incorporated metallic impurities, filtrating the insolubles such as a gelled polymer, and charging the filtrate (polybenzimidazole solution) into a poor solvent against the polybenzimidazole material. The conclusion reached by the inventor was that this procedure could not lower the above-defined total metallic concentration to the required level.